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Aphids, also known as greenfly or plant lice, are minute plant-feeding insects.
About 4,000 species of aphids are known, classified in 10 families; of these, around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners. They vary in size from 1-10 mm long.
Important natural enemies include the predatory ladybeetles (Coleoptera: Coccinellidae), hoverfly larvae (Diptera: Syrphidae), and lacewings (Neuroptera: Chrysopidae), and entomopathogenic fungi like Lecanicillium lecanii and the Entomophthorales.
Aphids are distributed world-wide, but they are most common in temperate zones. It is possible for aphids to migrate great distances (mainly through passive dispersal riding on winds) depending on the weather patterns; for example, the lettuce aphid spreading from New Zealand to Tasmania. They have also been spread by human transportation of infested plant materials.
Aphids, are in the superfamily Aphidoidea in the homopterous division of the order Hemiptera. Recent classification within the Hemiptera has changed the old term 'Homoptera' to two suborders: Sternorrhyncha (aphids, whiteflies, scales, psyllids...) and Auchenorrhyncha (cicadas, leafhoppers, treehoppers, planthoppers...) with the suborder Heteroptera containing a large group of insects known as the 'true bugs'; gnat bugs, pond skaters, shore bugs, toad bugs, water boatmen, backswimmers, etc. More recent reclassifications have resulted in a substantial rearrangement of the constituent families with the Aphidoidea, with some old families reduced to subfamily rank (e.g., Eriosomatidae), and numerous old subfamilies elevated to family rank.
Aphids contain sucking mouthparts called stylets. They have soft bodies; long, thin legs; two-jointed, two-clawed tarsi; and usually a pair of cornicles, abdominal tubes through which a waxy secretion is exuded. Aphids have two compound eyes and two ocular tubercles made up of three lenses, each of which is located behind and above the compound eyes. When host plant quality becomes poor or is crowded, female aphids will produce winged offspring that can disperse to other food sources.
Many, but far from all, aphids are monophagous (i.e. feeding only on 1 species of plant). Others, like Myzus persicae feed on hundreds of plant species across many families.
Similarly to related families, aphids passively feed on sap of phloem vessels in plants. This sap being kept under high pressure, once a phloem vessel is punctured, it is forced into the food canal. As they feed, aphids often transmit plant viruses to their food plants. These viruses can sometimes kill the plants.
Some species of ants "farm" aphids, protecting them on the plant they eat, and eating the honeydew that the aphids secrete; this is a mutualistic relationship. Aphid honeydew is rich in carbohydrates, of which the aphids ingest an excess, being phloem-feeders. Many aphids are host to endosymbiont bacteria, Buchnera, which live in specialized cells called bacteriocytes inside the aphid. These bacteria synthesize some essential amino acids that are absent in the phloem that the aphids eat.
Aphids are known for having unusual reproductive adaptations. Aphids undergo cyclical parthenogenesis. In the spring and summer, only females are present in the population. Reproduction is typically parthenogenetic and viviparous. Females undergo a modified meiosis that results in eggs that are genetically identical to their mother (parthenogenetic). The embryos develop within the mothers ovarioles, and give live birth to 1st instar nymphs (viviparous). Aphids typically live from 20-40 days and thus undergo multiple parthenogenetic, viviparous generations each summer. In the fall, a change in photoperiod and temperature cause females to parthenogenetically produce sexual females and males. The males are genetically identical to their mothers except they have lost one sex chromosome. Sexual females and males mate and females lay eggs that will develop outside of the mother. Thus in the fall aphids undergo sexual, oviparous reproduction. The aphids will overwinter as eggs and hatch out as females in the following spring.
Aphids have been known to have what is called telescoping generations. The parthenogenetic, viviparous female aphid will have a daughter within her who is already parthenogenetically producing its own daughter at the same time. This leads to the situation where the diet of a female aphid can have inter-generational effects on the body size and birth rate of aphids. In other words, what the aphid eats can directly change the size and fertility of the aphid's daughters and granddaughters (Nevo and Coll 2001, Jahn et al. 2005).
The following brief summary of what takes place in the plant-louse of the rose (Aphis rosae), may be regarded as typical of the family, though exceptions occur in other species: Eggs produced in the autumn by fertilized females remain on the plant through the winter and hatching in the spring give rise to female individuals which may be winged or wingless. From these, females are born parthenogenetically: that is to say, without the intervention of males, and by a process that has been compared to internal budding, large numbers of young resembling their parents in every respect except size are produced, which themselves reproduce their kind in the same way. This process continues throughout the summer, generation after generation being produced until the number of descendants from a single individual of the spring-hatched brood may amount to many thousands. In the autumn winged males appear; union between the sexes takes place and the females lay the fertilized eggs which are destined to carry the species through the cold months of winter. If, however, the food-plant is grown in a glasshouse or greenhouse where protection against cold is afforded, the aphids may go on reproducing agamogenetically (asexually) without cessation for many years. Likewise, in warm and tropical areas or during the growing season, aphids reproduce asexually without interruption. Since the young can become adults and reproduce within a few days, this process leads to the build-up of very large populations responsible for severe damage to crops and important economic losses; such populations often require pest control.
Not the least interesting features connected with this strange life-history are the facts that the young may be born by the oviparous or viviparous methods and either gamogenetically or agamogenetically, and may develop into winged forms or remain wingless, and that the males only appear in any number at the close of the season. Although the factors which determine these phenomena are not clearly understood, it is believed that the appearance of the males is connected with the increasing cold of autumn and the growing scarcity of food, and that the birth of winged females is similarly associated with decrease in the quantity or vitiation of the quality of the nourishment imbibed. Sometimes the winged females migrate from the plant they were born on to start fresh colonies on others often of quite a different kind (host plant alternancy). Thus the apple aphid (Aphis mali) after producing many generations of apterous females on its typical food-plant gives rise to winged forms which fly away and settle upon grass or corn-stalks.
Another widespread aphid in the Phylloxeridae family is Daktulosphairia vitifoliae (still frequently called by its former name Phylloxera vitifoliae), the famous Phylloxera which causes enormous loss by attacking the leaves and roots of grape vines. Its life-history is similar to that of Aphis rosae described above. In the autumn a single fertile egg is laid by apterous females in a crevice of the bark of the vine where it is protected during the winter. From this egg in the spring emerges an apterous female who makes a gall in the new leaf and lays therein a large number of eggs. Some of the apterous young that are hatched from these form fresh galls and continue to multiply in the leaves, others descend to the root of the plant, becoming what are known as root-forms. These, like the parent form of spring, reproduce parthenogenetically, giving rise to generation after generation of egg-laying individuals. In the course of the summer, from some of these eggs are hatched females which acquire wings and lay eggs from which wingless males and females are born. From the union of the sexes comes the fertile egg from which the parent form of spring is hatched.
Some species of cabbage aphids (like Brevicoryne brassicae) reproduce rapidly during the summer. They are all females, and can produce up to 41 generations of offspring. If no aphids had died during the summer, there would be more than one and a half billion billion billion aphids (1.5 x 1027) by the end of the season.
Aphids probably first appeared around 280 million years ago, in the Carboniferous period. They probably fed on plants like Cordaitales or Cycadophyta. The oldest known aphid fossil is one of the species Triassoaphis cubitus from the Triassic. There were relatively few species of aphids at that time, and the number of species only considerably increased since the appearance of angiosperms 160 millions of years ago. This is due to the fact that angiosperms provide an occasion for aphids to become specialized.
Aphids have not always looked like they do nowadays. Organs like the cauda or the siphunculi were not evolved until the Cretaceous.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aphid". A list of authors is available in Wikipedia.|